In recent years, magnetic heads utilizing a giant magneto resistive (GMR) effect have been used as magnetic heads for reproduction in high density magnetic recording systems.
According to a method for sensing magnetism in such a magnetic head, a constant sensing current is made to flow through a GMR element where a non-magnetic conductive layer is placed between a magnetic layer (pinned layer) of which the direction of magnetization is fixed, and a magnetic layer (free layer) of which the direction of magnetization is free so that a phenomenon is utilized, wherein the potential difference of the GMR element changes in response to a change in the magnetic flux that leaks from the magnetic recording medium in which a magnetic recording signal has been written at a high density.
Accordingly, though a large output can be gained in accordance with the Ohm's law in the case where a large amount of current is made to flow, there is a limit in the amount of sensing current that can actually flow, due to the emission of heat and the like.
In recent years, research has been progressing on magnetic heads utilizing a spin tunneling magneto resistive (TMR) effect, which are magnetic heads for reproduction, and will substitute for such GMR elements of the next generation (see, for example, Japanese Unexamined Patent Publication 2002-237628).
Such a TMR element has a structure where an insulating layer, such as Al-oxide, is sandwiched between a magnetic layer (pinned layer) of which the direction of magnetization is fixed, and a magnetic layer (free layer) of which the direction of magnetization is free, and in the case where a sensing current is made to flow perpendicular to the surface of a magnetic sensing film, the tunnel current that passes through the insulating film changes in accordance with the angle made between the directions of magnetization of the two magnetic layers, so that this change can be read out as a change in the potential difference of the TMR element.
A large output can be expected from such a TMR element because an extremely large resistance changing ratio such as, for example, several tens of % can be gained.
However, an insulating film, such as Al-oxide, is inserted in a TMR element, and therefore, the resistance value is too large to allow a large sensing current to actually flow, and accordingly, a problem arises that a large output cannot be gained.
In addition, there is a defect in that there is a large amount of noise due to the large resistance, and therefore, in the case where the output of the TMR element is amplified, noise is also greatly amplified, preventing the detection of a microscopic magnetic flux from being carried out with a high level of precision.
On the other hand, the usage of an MR element having a low level of detection sensitivity has been proposed wherein the MR element is incorporated in a Hartley-type oscillating circuit, or a Colpitts-type oscillating circuit, so as to convert the change in the resistance of the MR element into a change in the reactance, and thereby, the existence of a high frequency signal is detected (see, for example, Japanese Unexamined Patent Publication H6(1994)-338005).
In the case of the above described Japanese Unexamined Patent Publication H6(1994)-338005, however, an MR element forms a part of an LC circuit: that is to say that the MR element forms a resonance part of an oscillating circuit, and therefore, a problem arises that the oscillation becomes unstable when the resistance of the MR element changes.
Therefore, though no actual problems arise in the case where an error of approximately 10−4 to 10−5 is allowed, such as in a video cassette recorder for recording image data, a problem arises that an MR element is not suitable for a sensing method that requires an error rate of approximately 10−7 or less, such as in a hard disk drive.
In addition, an MR element forms a resonance circuit, and therefore, there is a risk that a large amount of current flows through the MR element, and thereby, the element characteristics are changed or damaged.
In particular, in the case of a TMR element that has an insulating layer, and where an insulation breakdown becomes a problem, the above described circuit configuration is not practical.
Accordingly, an object of the present invention is to overcome the restriction in the level of sensitivity due to the upper limit of the sensing current that can be made to flow.